The present invention relates to phonograph cartridges and more particularly to a replaceable stylus assembly for use in phonograph cartridges.
Phonograph pickup cartridges are generally either of the magnetic or ceramic type. Magnetic cartridges have recognized advantages in fidelity and response characteristics over ceramic cartridges and are widely used in quality high fidelity and stereophonic equipment.
In a magnetic cartridge, a jeweled stylus is used to track the grooves of a record disc. The stylus has attached to it a magnetic member which moves with the stylus within a magnetic circuit. The circuit includes portions about which a voltage generating coil is mounted so that flux variations in the circuit resulting from movement of the magnetic member are reflected in voltage changes at the output of the coils.
Heretofore, magnetic cartridges have been classified as being of either the moving magnet, moving iron type, or moving coil type. In the former, typified by U.S. Pat. No. 3,297,831, a permanent magnet moves directly with the jeweled stylus. The latter construction is typified by U.S. Pat. No. 3,546,399 wherein a tubular armature of permeable material moves with the stylus. The armature is magnetically coupled to a fixed permanent magnet which thus serves as the flux source to the magnetic circuit through the armature. In either case, it is important that the magnetic member (i.e., permanent magnet in the first type of cartridge or permeable armature in the latter cartridge) move in faithful correspondence to the jeweled stylus.
Heretofore, it was common practice to support the armature within an elastomeric grommet which in turn was supported within a nonmagnetic tube. To support the gromment in position, the non-magnetic tube was crimped over the grommet for substantially the entire length of the grommet. Ideally, the elastomeric grommet was to act as a universal joint for the armature. However, because of the nature of the support of the grommet within the non-magnetic tube, the joint was not truly a universal joint and the mechanical impedance of the system was unduly high thus adversely affecting low and mid frequency responses of such prior art devices.
In U.S. Pat. No. 3,964,751, there is disclosed a stylus assembly which seeks to alleviate the above problem by having the grommet engage the non-magnetic tube along an edge. This is accomplished in accordance with the patent by tapering the tube inwardly toward the rear and positioning the grommet so that its rear edge would engage the tapered tube wall. While the results obtained with this assembly were superior to alternate constructions then available, true tracking of the record groove information was not entirely obtained.
It was observed that the stylus assembly disclosed in U.S. Pat. No. 3,964,751 did not respond in the same manner at all frequencies. In wedging the grommet into the tapered tube, the tube walls unavoidably compress portions of the grommet, thereby causing it to act as a spring. The mechanical hysteresis introduced into the system as a result of the spring action of the grommet affected the return action of the stylus differently at different frequencies and amplitudes.
In an ideal stylus, the return action (i.e., the force which tends to return the stylus to its zero position after the stylus has been displaced in response to an undulation in a record groove) should be independent of the damping function of the armature support grommet.
In view of the above, it is the principal object of the present invention to provide an improved phonograph stylus assembly wherein the mechanical parameters for compliance and damping are separated. Compliance is controlled by a tension-free tie wire which serves only to return the stylus to its zero position after displacement. Damping is controlled by an elastomeric grommet which damps out unwanted resonant vibrations.